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My rear derailleur took a knock from a fall, and the entire shifting range moved inwards by about one sprocket width. I suspect, but am not sure, that the mech hanger got slightly bent (could be completely wrong, not really sure how to tell).

I was able to counteract this by adjusting the limit screws, and the derailleur seems to be shifting well over the entire range. The way it's set up currently I can no longer push it into the spokes by hand (i.e. it's the limit screw and not cable tension that stops it).

I am tempted to just leave things are they are.

Is it OK to ride like this, or are there compelling reasons (safety, long-term reliability, etc) to figure out what got damaged and repair/replace it?

Here are some pics:

pic 1pic 2pic 3pic 4

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    I straightened out a bent hanger today, with a large pair of Channellock pliers and a buddy to hold the bike. It's something that any competent bike mechanic can do in 5 minutes, if the situation is not too odd.
    – Daniel R Hicks
    Commented Jul 19, 2018 at 22:05

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Derailleur hangers are usually aluminum, but they're pretty substantial and anything that could put significant stress on them while riding is likely to come from something that would be catastrophic even should the hanger survive the event totally unscathed. Things like a chain side plate coming loose and catching on the rear derailleur cage or the rear derailleur cage itself getting caught by a rear wheel spoke aren't going to end well even if nothing happens to the hanger at all.

I'd just remove the rear wheel, maybe unscrew the hanger itself from the frame (might not even need to remove the rear derailleur itself or even the chain), and inspect the hanger, looking for cracks or deformation beyond a simple bending of the main part of the hanger. For example, is the screw hole ovalized or otherwise distorted?

If there aren't any cracks nor any distortion, I'd ride on it.

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  • Thanks for the suggestions! I removed the hanger to take a look (and have added some pics to the question). It looks like it might be bend, but I'm not sure since I don't know what it's supposed to look like. I don't see anything that would hint at imminent catastrophic failure - would you agree?
    – NPE
    Commented Jul 19, 2018 at 17:32
  • @NPE It looks safe to me - with the caveat that small cracks won't be apparent in the photos you posted. You should be able to bend it straight(er), too. Clamp the thin part in a vise and carefully bend the hanger straight. Just be really careful whatever you clamp it in to straighten it doesn't mar the surface - things like the teeth on a vise might damage the surface in ways that lead to cracking over time. I've done it a couple of times. Given the way it's bent, though, also look for a replacement. The bending looks like it angles the RD into the wheel - you don't want the RD in spokes.
    – Andrew Henle
    Commented Jul 20, 2018 at 10:48
  • Based on this discussion, I've ordered a replacement. In the meantime, will ride the current one. Thanks again!
    – NPE
    Commented Jul 20, 2018 at 16:07
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Derailleur hangers are supposed to be, to some degree, a component that bends to save the derailleur. The hanger on one of my bikes seems to get bent at the slightest impact. I'm considering ordering a spare and carrying it around in my tool kit.

If the indexing was severely affected by the fall (moving inboard a whole sprocket definitely counts as 'severe'), you need to address the problem properly. A bent hanger is going to affect shifting and put extra strain on the chain, and you risk the cage hitting the spokes when changing to the largest sprocket.

You can eyeball the derailleur cage alignment by looking down the chain-line from the back of the bike - see image below from this question. The derailleur cage should appear to be parallel to the chainrings.

My LBS only charges $20 for hanger alignment and indexing adjustment (I'm in the USA). Worth it to get the hanger straight and avoid associated problems.

enter image description here

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  • Thanks for the nice picture and for the explanation. Looking at mine, the derailleur cage does look like it's leaning inwards (I'm struggling to take a good picture). Now sure this is useful, but I took some pictures of the hanger and have added them to the question.
    – NPE
    Commented Jul 19, 2018 at 17:54
  • I wouldn't worry too much about slightly angling the derailleur affecting shift performance - the chain is flexible enough that the slight angle shouldn't really matter, as the limit screws and the barrel adjuster should be able to line up the derailleur enough to make shifting work just fine. What's more worrying to me is the pictures look like the bent hanger will angle the derailleur towards the wheel - perhaps enough to make the derailleur contact the spokes when shifting onto the big cog. That will cause a lot more damage than a bent hanger, and cost a lot more than $20 to fix.
    – Andrew Henle
    Commented Jul 20, 2018 at 10:53
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It was the late 80s or very early 90s when replaceable rear derailleur hangers (yeah, yeah, I know...mech) first made their way onto mtn bikes. At that time the rule was that even with slight deformations the hanger should be replaced. It is aluminum, and one should assume that there is a crack, or weakening of the aluminum, as it is among the least stable of the crystalline structured metals. Though Park tool as well as other tool manufacturers sell derailleur hanger alignment tools, and there are plenty of vids showing how to re-bend/align derailleur hangers, I always replace them. Though I will certainly re-bend to get home/finish a ride, PLEASE replace bent hangers. At around $10.00 US, its just not worth the risk of that hanger suffering a catastrophic failure while riding, as you can be sure that upon breaking the hanger,derailleur,and chain are swinging immediately into your rear wheel, and you will likely suffer a nasty crash. Typically upon buying a new bike, I also buy a back up hanger in case I am unable in the future to find that hanger. May peace be upon you, and much love to all. :)

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  • In theory, what you say is true. However, a lot of hangars are beefy enough that they can be bent back once. It's true that multiple bends will cause work hardening and fracture, however.
    – Weiwen Ng
    Commented Aug 11, 2023 at 18:01
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There's some theoretical safety risk in that if your gears are skipping around or not engaging totally reliably, the bike could not do what you want it to at just the wrong time.

If having a shop fix it with a hanger alignment tool isn't in the cards for whatever reason, then you can probably make it better than it is now by poking a wrench into the derailer pivot bolt to use as a lever arm and aligning the hanger by eye that way, eyeballing the parallelism between the pulleys and the cogs in both axes. The derailer won't be affected. This is a lot better than leaving it as-is.

Whenever one aligns a replaceable hanger, there's some distant risk of it breaking, but it's rare and usually more goes along with a more severely bent or visibly cracked one.

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